A typical router or switch configuration includes a chassis which contains basic components such as power supply, fans, and multiple slots which have interface cards and network modules. The linecards (interface cards and network modules) which are inserted into slots handle packet ingress and egress and other networking functions. Line cards provide one or more interfaces over which traffic flows. Thus, depending on the number of slots and interfaces, a switch or a router can be configured to work with a variety of networking protocols. Some switches are fixed configuration switches that do not use line cards to implement interfaces.
A switch includes a central forwarding engine, the supervisor, and switches utilizing line cards may implement the distributed learning and forwarding model where linecards that are distributed-forwarding-enabled make forwarding decisions locally, leaving the supervisor engine free to perform routing and management functions. Distributed Forwarding requires that the Layer 2 (L2) Medium Access Control (MAC) address tables exist on the supervisor and multiple linecards in the system.
Linecards and the supervisor build address tables by examining the source address on each incoming packet and associating the source address with the port on which the packet was received. When a packet is received its destination address is looked up in the address table and, if there is a hit, the linecard outputs the packet on the port associated with the looked-up address.
A flow is asymmetrically routed if the transmit path from A to B is different from the return path from B to A, where A and B represent ports in the switch or router.